Basically, the line about the car being "no Tomcat" was used as further fodder in the never-ending debate over whether or not Harm and Mac's connection is love, in the romantic sense, or just a deep un-romantic friendship.
To me, I think she's expressing her doubts not only about her "new life," but about her "new man," too - both in comparison to JAG and Harm.
IMHO, Harm shows a genuine sort of regret as she leaves, that look in his eyes after she has driven away says he let her get away. I've heard it argued that the whole thing for Harm was just jealousy over Dalton's money, lifestyle, etc. Maybe it was; maybe that's not all it was.
Whatever side of the argument you come down on, you've got to admit that for a woman "in love" with Dalton Lowne, she certainly didn't look too excited about leaving with him that day. See, my little shipper heart is not totally stilled. It just needs a good shot of emotional adrenaline to get going. LOL!
KAL - inside, please